In the field of digital radiography a wide variety of image acquisition techniques have been developed rendering a digital representation of a radiographic image.
Among such techniques are computerised tomography, nuclear magnetic resonance, ultrasound detection, detection of a radiation image by means of a CCD sensor or a video camera, radiographic film scanning etc.
In still another technique a radiation image, for example an X-ray image of an object, is stored in a screen comprising a photostimulable phosphor such as one of the phosphors described in European patent publication 503 702, published on 16 Sep. 1992 and U.S. Ser. No. 07/842,603 now U.S. Pat. No. 5,340,661. The technique for reading out the stored radiation image consists of scanning the screen with stimulating radiation, such as laser light of the appropriate wavelength, detecting the light emitted upon stimulation and converting the emitted light into an electric representation for example by means of a photomultiplier and finally digitizing the signal.
One of the benefits of a digital radiographic system resides in the possibility of processing the digital image representation before display or hard copy recording. The term "processing" in this context means any kind of image-processing such as noise filtering, contrast enhancement, data compression etc.
In some digital imaging systems the same original has to be processed in different ways to produce multiple hard-copies or displays originating from the same image. These different versions of one image may be helpful for a radiologist in making a specific diagnosis.
For example different versions of one original image may be generated that are processed taking into account different contrast enhancing modifying curves or different window level settings etc. Time-consuming, mostly convolution or non-linear neighbourhood operations, such as unsharp masking for edge enhancement, have to be repeated if e.g. several images processed according to different kernel sizes have to be produced.
A digital radiographic image is commonly represented by about 10 MB digital data. The computation time required for processing such an amount of data may extend to orders of minutes per processing cycle.
Hence, in case multiple differently processed versions of one image are required, it is highly desirable to optimize the processing procedure as far as the computation time is concerned.
Recently a new image processing technique has been developed. According to this technique an image (more specifically a digital signal representation of an image) is first decomposed into a multiresolution representation which represents localised image detail at multiple scales. For example, the image is decomposed into a sequence of detail images at multiple resolution levels and a residual image at a resolution lower than the minimum of said multiple resolution levels. Next, the pixel values of said multiresolution representation are modified by means of modifying function. And finally a processed image is computed by applying a reconstruction algorithm to the modified multiple resolution representation, the reconstruction algorithm being such that if it were applied to the unmodified multiresolution representation then said original image or a close approximation thereof would be obtained.
In a preferred embodiment the decomposition is pyramidal, meaning that the number of pixels in the components at successive resolution levels decreases.